1. Field of the Invention
The present invention relates to a power supply for an ac metal halide lamp, and specifically to such a supply wherein a commutator is used to supply regulated dc power to the lamp with alternating polarity.
2. Description of the Prior Art
Metal halide lamps have gained considerable popularity as light sources for television, photographic and theater applications. Such metalogen lamps have a radiation temperature similar to daylight, good color rendering characteristics, high luminous efficiency and small arc dimensions.
These favorable characteristics are achieved by using exact amounts of the rare earths dysprosium, holmium and thulium supplemented by bromine, iodine and mercury. Because of this make-up, metal halide lamps have rigorous power supply requirements. First, a high voltage pulse is required for ignition. After ignition, but while still cold, the lamps can operate as half wave rectifiers. In that operational state, which normally lasts less than 1 second, the supplied power must not exceed a certain peak current value typically on the order of 70 amperes.
During lamp warm up, which may last several minutes, the lamp voltage is considerably lower than after the operating temperature has been reached. For example, a lamp rated at 95 volts after warm up may operate at about 20 volts during the warm up period. Furthermore, the maximum lamp current during warm up may be considerably higher (typically 13 amperes) than during later operation. (typically 6.95 amperes). Both during warm up and after, a certain maximum lamp power (typically on the order of 660 watts) cannot be exceeded without damaging the lamp.
The metal halide lamps require ac operation. Thus both the warm up and normal operating voltages and currents must be provided by an ac power supply. Moreover, it is desirable to provide constant power to the lamp, so as to insure the constant light output and color temperature required for photographic and television applications.
In the past, these power supply requirements have necessitated the use of special inductive ballasts in which the voltage to current ratio is constant within small tolerances, and is barely influenced by temperature changes and a magnetic environment. Such power supplies are very heavy, owing to the considerable weight of the inductive ballast itself. A weight of 150 pounds is not uncommon. This is a serious disadvantage. Although the lamp itself is very light, its portability is seriously impaired by the heavy weight of the power supply.
In the prior art ballast-type supplies, a series choke or inductor is used wherein the voltage is dropped in accordance with the amount of current drawn, so as to regulate the power output. This requires a differential between the rated lamp voltage and the line voltage. For a metal halide lamp that is rated at 95 volts, a 220 volt source typically must be used. Thus to operate from a 110 volt ac line, a step up transformer must be used in addition to the ballast inductor. Further, since the inductor is adapted for operation at a particular input voltage, taps must be provided to compensate for different line voltages. All of these factors add to the cost, weight and complexity of the power supply, and may lead to reduced efficiency.
An object of the present invention is to provide a power supply for a metal halide lamp which does not require a ballast inductor. Another object is to provide a power supply in which a regulated ac output is obtained by the cooperation of a dc regulator and a commutator which provides the regulated dc signal to the load with alternating polarity. Such a power supply is considerably lighter than those of the prior art, as well as being lower in cost and highly efficient.